Utilizing mutants of mammalian cells and E. coli we shall study the regulation of de novo purine biosynthesis, and the interrelationship between purine salvage enzymes and de novo purine biosynthesis. This will be done by selectively isolating regulatory mutants. We have previously found that glutamine concentration affect de novo purine biosynthesis differently in HGPRT minus mutants than wild type. This glutamine effect will be studied in more detail. Moreover preliminary experiments indicate that HGPRT minus cells may have a more "relaxed" phenotype, then wild type. We shall attempt to relate this to the presence of an unusual polynucleotide polyphosphates present in the cell. Although actinomycin D inhibits de novo purine biosynthesis (our data), inhibitors of protein synthesis do not. The mechanism of inhibition by actinomycin D will be investigated. We shall characterize further already isolated mutants.